Electrochemical Lithium Storage Performance of Molten Salt Derived V_(2)SnC MAX Phase

MAX phases are gaining attention as precursors of two-dimensional MXenes that are intensively pursued in applications for electrochemical energy storage.Here,we report the preparation of V_(2)SnC MAX phase by the molten salt method.V_(2)SnC is investigated as a lithium storage anode,showing a high gravimetric capacity of 490 mAh g−1 and volumetric capacity of 570 mAh cm^(−3) as well as superior rate performance of 95 mAh g^(−1)(110 mAh cm^(−3))at 50 C,surpassing the ever-reported performance of MAX phase anodes.Sup-ported by operando X-ray diffraction and density functional theory,a charge storage mechanism with dual redox reaction is proposed with a Sn-Li(de)alloying reaction that occurs at the edge sites of V_(2)SnC particles where Sn atoms are exposed to the electrolyte followed by a redox reaction that occurs at V_(2)C layers with Li.This study offers promise of using MAX phases with M-site and A-site elements that are redox active as high-rate lithium storage materials.

A review of novel ternary nano-layered MAX phases reinforced AZ91D magnesium composite

In recent decades, the demand for lightweight and high specific strength materials brings about the development of magnesium matrix composites. Different from some traditional binary ceramic particles, such as SiC, Al_(2)O_(3), the novel ternary nano-layered M_(n+1)AX_(n)(MAX)phase carbide or nitride ceramics exhibit metal-like properties and self-lubricate capacity(where “M” is an early transition metal, “A” belongs to the group A element, “X” is C or/and N, and n = 1–3). Ti_(2)AlC, as the representative of the MAX phase, was interestingly introduced into the magnesium matrix. Layered Ti_(2)AlC MAX phased reinforced AZ91D magnesium composites manufactured through the stir casting exhibit sufficient deformation capacity due to unique deformation behaviors of MAX, namely delamination and the formation of kinking band. Further,the Ti_(2)AlC-AZ91D composites exhibit a distinctive characteristic in strengthening mechanism, damping mechanism and tribological capacity due to the other special properties of MAX phase, such as self-lubricated property. Accordingly, to give a comprehensive understanding, we overviewed the fabrication process, microstructural characterization, mechanical properties, damping property and tribological capacity on these composites. In order to understand the A-site effect in MAX phase on the microstructure, we introduced another representative Ti_(3)SiC_(2)MAX phase to explain the interfacial evolution. In addition, due to the high aspect ratio of MAX, MAX particles could be orientationally regulated in Mg matrix by plastic deformation such as hot extrusion. Herein, we discussed the anisotropic mechanical and physical properties of the textured composites produced by hot extrusion. Moreover, the potential applications and future development trends of MAX phases reinforced magnesium matrix composites were also given and prospected.

Synthesis and Characterization of Magnetic MAX Phase(Cr2-xMnx)GaC

The solid solution of (Cr2-xMnx)GaC with magnetic properties was synthesized by pressureless sintering.The composition,morphology,and magnetic properties of products were characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM),and vibrating sample magnetometer (VSM).The experimental results indicate that the solid solubility of Mn is related to the value of x,which reaches the maximum at x=0.4 and the characteristic peaks shift effect is most obvious.After the solution treatments,the samples of (Cr2-xMnx)GaC still presents the layered structure of MAX phase,and the lattice parameter has decreased slightly.By characterizing the magnetic properties of (Cr2-xMnx)GaC,the successful doping of Mn atoms was confirmed,and the intensity of magnetism was positively correlated with the doping amounts of Mn.

New ordered MAX phase Mo_2 TiAlC_2: Elastic and electronic properties from first-principles

First-principles computation on the basis of density functional theory(DFT) is executed with the CASTEP code to explore the structural, elastic, and electronic properties along with Debye temperature and theoretical Vickers’ hardness of newly discovered ordered MAX phase carbide Mo2TiAlC2. The computed structural parameters are very reasonable compared with the experimental results. The mechanical stability is verified by using the computed elastic constants. The brittleness of the compound is indicated by both the Poisson’s and Pugh’s ratios. The new MAX phase is capable of resisting the pressure and tension and also has the clear directional bonding between atoms. The compound shows significant elastic anisotropy. The Debye temperature estimated from elastic moduli(B, G) is found to be 413.6 K. The electronic structure indicates that the bonding nature of Mo2TiAlC2is a mixture of covalent and metallic with few ionic characters. The electron charge density map shows a strong directional Mo–C–Mo covalent bonding associated with a relatively weak Ti–C bond.The calculated Fermi surface is due to the low-dispersive Mo 4d-like bands, which makes the compound a conductive one.The hardness of the compound is also evaluated and a high value of 9.01 GPa is an indication of its strong covalent bonding.

Solid solution strategy modulated defects engineering of(Cr_(1-x)V_(x))_(2)AlC MAX phase toward superior electromagnetic wave absorption

Defects engineering is an effective strategy for manipulating electromagnetic parameters and enhancing electromagnetic wave(EMW)absorption capacity.However,the relationship between them is not clear,especially in solid solution structures.In this work,a series of(Cr_(1-x)V_(x))_(2)AlC MAX phase solid solutions with layered structure were prepared via tuning the ratio of Cr and V to explore their EMW absorption performance.The experimental results indicated that the doping of V atoms at the M-site could effectively regulate its impedance matching and EMW absorption properties by introducing appropriate numbers of defects in the crystal,such as twin boundaries,dislocations and lattice distortions.Among them,if Cr:V=3:1,Cr_(1.5)V_(0.5)AlC,as radar absorption materials,could reach a strong reflection loss of-51.8 dB at the frequency of 12.8 GHz under an ultra-thin thickness of 1.3 mm.The reflection loss value could attain-10 dB in a wide frequency range of 2.7-18 GHz and thickness range of 1-5 mm.In addition,after high temperature and acid-alkali immersion treatment,this sample still had good EMW absorption capability,and the effective absorption bandwidth was enhanced from 2.3 to 2.6 GHz after concentrated acid immersion or 3.1 GHz after concentrated alkali immersion.This work has great reference significance for the research and development of high-performance MAX-based EMW absorption materials in harsh environments.

Factors influencing synthesis and properties of MAX phases

MAX phases are a member of ternary carbide and nitride,with a layered crystal structure and a mixed nature of chemical bonds(covalent-ionic-metallic)that promote MAX phases embracing both ceramic and metal characteristics.As a result,MAX phase ceramics emerge with remarkable properties unique from other traditional ceramics.In this review,we focus on alternate processing approaches for MAX phases that are cost-effective and energy-saving.The MAX phase purity,formation of other unwanted phases,microstructure,and properties are influenced by many parameters during processing.Therefore,we highlight the effect of numerous factors,which alternately diminish the efficiency and performance of materials.Here,the impact of several parameters,such as starting materials,stoichiometric composition,temperature,pressure,particle size,porosity,microstructure,mechanical alloying,mechanical activation,ion irradiation,and doping,are summarized to reveal their influence on the synthesis and properties of MAX phases.The potential applications of MAX phases are considered for their development on a commercial scale toward the industry.

Breaking the trade-off of hardness–ductility in(Cr_(1−x)Mo_(x))_(2)AlC MAX phase coatings via a hierarchical structure

The Cr_(2)AlC MAX phase offers a remarkable combination of excellent electrical conductivity and hot corrosion resistance in extremely harsh environments.However,the strong trade-off between hardness and toughness is rather limited by its nanolaminate structure for desired applications.Taking the solid solution strengthening and gradient hardening synergy,in this work,high-purity Cr_(2)AlC coatings with various Mo solid solutions were successfully fabricated via a hybrid sputtering technique followed by subsequent annealing.Interestingly,gradually changing the Mo concentration in the(Cr_(1−x)Mo_(x))_(2)AlC(x=0.05–0.24)coating enabled a hierarchical structure responsible for gradient refinement of the crystal grain size,and the solid solution of Mo atoms at Cr sites and the gradient variation in the Mo content were confirmed via the atomic-resolution transmission electron microscopy(TEM)characterization.Compared with those of the pristine Cr_(2)AlC coating,the nanoindentation hardness and toughness values of H/E and H_(3)/E_(2) for the hierarchical(Cr_(1−x)Mo_(x))_(2)AlC coating were enhanced by approximately 26%,12%,and 57%,respectively.On the basis of comprehensive experiments and ab initio simulations,the reasons behind this observation were mainly attributed to the synergistic effect of Mo occupancy with strong bonding at the Cr site and the strengthening of grain refinement induced by the gradient Mo concentration in the(Cr_(1−x)Mo_(x))_(2)AlC coating.These findings not only reveal the underlying mechanism for the Mo solid solution in the Cr_(2)AlC coating but also offer a new concept for developing ultrahigh-strength ductility materials for the laminar MAX phase.

One-dimensional core-sheath Sn/SnO_(x)derived from MAX phase for microwave absorption

One-dimensional(1D)metals are highly conductive and tend to form networks that facilitate electron hopping and migration.Hence,they have tremendous potential as microwave-absorbing(MA)materials.Traditionally,1D metals are mainly precious metals such as gold,silver,nickel,and their preparation methods often have low yield and are not environmentally friendly,which has limited the exploration in this area.Herein,the unique nanolaminate structure and chemical bond characteristics of Ti_(2)SnC MAX phase is successfully taken advantages for large-scale preparation of Sn whiskers,and then,core-sheath Sn/SnO_(x)heterojunctions are obtained by simply annealing at different temperatures.The heterojunction annealed at 500℃possesses favorable MA performance with an effective absorption bandwidth of 5.3 GHz(only 1.7 mm)and a minimum reflection loss value of51.97 dB;its maximum radar cross section(RCS)reduction value is 29.59 dB·m^(2),confirming its excellent electromagnetic wave attenuation ability.Off-axis electron holography is used to visually characterize the distribution of charge density at the cylindrical heterogenous interface,confirming the enhanced interfacial polarization effect.Given the diversity of MAX phases and the advantages of the fabrication method(e.g.,green,inexpensive,and easily scalable),this work provides significant guidance for the design of 1D metal-based absorbers.

Q-switched giant pulsed erbium-doped all-fiber laser with V_(2)ZnC MAX phase saturable absorber

MXenes,drawn from MAX phases,are special two-dimensional substances with numerous advantages in nonlinear optics,specifically in giant and ultrashort pulsed-laser applications.Ti_(3)C_(2)T_(x)and Ti_(2)CT_(x)nanosheets however rapidly deteriorate under ambient conditions,limiting their applications.This paper demonstrates how excellent modulation depth of one of the MAX phase compounds vanadium zinc carbide(V_2ZnC)makes it a brilliant saturable absorber(SA)in passively Q-switched all-fiber pulsed lasers,integrated such that a 16.73-μm V_(2)ZnC-polyvinyl alcohol(PVA)thin film acts as SA in the laser.Saturable and non-saturable absorptions were found to be 13.2%and 10.47%,while saturation optical intensity and modulation depth were 6.25 k W/cm^(2)and 12.43%,respectively,illustrating the optical nonlinearity.The superiority of MAX-PVA,fabricated in four distinct ratios,was demonstrated by the fact that it self-starts a giant pulsed laser at pump power as low as 22.5 mW and firmly accomplished 120.6 kHz repetition rate with a pulse width of 2.08μs.It is a fine SA for the use of pulsed-laser production using all-fiber laser due to fabrication simplicity and great optical,thermophysical,and mechanical qualities.

Preparation of Ti_2AlC MAX Phase Coating by DC Magnetron Sputtering Deposition and Vacuum Heat Treatment

Due to the excellent corrosion resistance and high irradiation damage resistance,Ti 2AlC MAX phase is considered as a candidate for applications as corrosion resistant and irradiation resistant protective coating.MAX phase coatings can be fabricated through firstly depositing a coating containing the three elements M,A,and X close to stoichiometry of the MAX phases using physical vapor deposition,followed by heat treatment in vacuum.In this work,Ti-Al-C coating was prepared on austenitic stainless steels by reactive DC magnetron sputtering with a compound Ti （50）Al （50） target,and CH4 used as the reactive gas.It was found that the as-deposited coating is mainly composed of Ti 3AlC antiperovskite phase with supersaturated solid solution of Al.Additionally,the ratio of Ti/Al remained the same as that of the target composition.Nevertheless,a thicker thermally grown Ti 2AlC MAX phase coating was obtained after being annealed at 800℃ in vacuum for 1 h.Meanwhile,the ratio of Ti/Al became close to stoichiometry of Ti 2AlC MAX phases.It can be understood that owing to the higher activity of Al,it diffused quickly into the substrate during annealing,and then more stable Ti 2AlC MAX phases transformed from the Ti 3AlC antiperovskite phase.

From structural ceramics to 2D materials with multi-applications:A review on the development from MAX phases to MXenes

MAX phases(Ti_(3)SiC_(2),Ti_(3)AIC_(2),V_(2)AlC,TiqAlN_(3),etc.)are layered ternary carbides/nitrides,which are generally processed and researched as structure ceramics.Selectively removing A layer from MAX phases,MXenes(Ti_(3)C_(2),V_(2)C,Mo_(2)C,etc.)with two-dimensional(2D)structure can be prepared.The MXenes are electrically conductive and hydrophilic,which are promising as functional materials in many areas.This article reviews the milestones and the latest progress in the research of MAX phases and MXenes,from the perspective of ceramic science.Especially,this article focuses on the conversion from MAX phases to MXenes.First,we summarize the microstructure,preparation,properties,and applications of MAX phases.Among the various properties,the crack healing properties of MAX phase are highlighted.Thereafter,the critical issues on MXene research,including the preparation process,microstructure,MXene composites,and application of MXenes,are reviewed.Among the various applications,this review focuses on two selected applications:energy storage and electromagnetic interference shielding.Moreover,new research directions and future trends on MAX phases and MXenes are also discussed.

Theoretical prediction,synthesis,and crystal structure determination of new MAX phase compound V2SnC

Guided by the theoretical prediction,a new MAX phase V2SnC was synthesized experimentally for the first time by reaction of V,Sn,and C mixtures at 1000°C.The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations,X-ray diffraction(XRD),energy dispersive X-ray spectroscopy(EDS),and high resolution scanning transmission electron microscopy(HR-STEM).The stacking sequence of V2C and Sn layers results in a crystal structure of space group P63/mmc.The a-and c-lattice parameters,which were determined by the Rietveld analysis of powder XRD pattern,are 0.2981(0)nm and 1.3470(6)nm,respectively.The atomic positions are V at 4f(1/3,2/3,0.0776(5)),Sn at 2d(2/3,1/3,1/4),and C at 2a(0,0,0).A new set of XRD data of V2SnC was also obtained.Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy,satisfied Born-Huang criteria of mechanical stability,and positive phonon branches over the Brillouin zone.It also has low shear deformation resistance c44(second-order elastic constant,cij)and shear modulus(G),positive Cauchy pressure,and low Pugh’s ratio(G/B=0.500<0.571),which is regarded as a quasi-ductile MAX phase.The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.

In-situ growth of MAX phase coatings on carbonised wood and their terahertz shielding properties

Electromagnetic interference(EMI)shielding materials have received considerable attention in recent years.The EMI shielding effectiveness(SE)of materials depends on not only their composition but also their microstructures.Among various microstructure prototypes,porous structures provide the advantages of low density and high terahertz wave absorption.In this study,by using carbonised wood(CW)as a template,1-mm-thick MAX@CW composites(Ti2AlC@CW,V2A1C@CW,and Cr2AlC@CW)with a porous structure were fabricated through the molten salt method.The MAX@CW composites led to the formation of a conductive network and multilayer interface,which resulted in improved EMI SE.The average EMI SE values of the three MAX@CW composites were>45 dB in the frequency of 0.6-1.6 THz.Among the composites,V2A1C@CW exhibited the highest average EMI SE of 55 dB.

Synthesis and electromagnetic wave absorption performances of a novel(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)high-entropy MAX phase

Herein,a novel kind of high-entropy MAX phases,(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders were success-fully synthesized by a newly proposed two-step solid state reaction process.The oxidation experiments demonstrate that the oxidation products of Al_(2)Mo_(3)O_(12) and rutile TiO 2 are formed at about 600 and 800℃,respectively.Besides,the dielectric and electromagnetic(EM)wave absorption properties of(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders and those after oxidation at different temperatures were also exam-ined.The results show that the as-synthesized(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders possess excellent EM wave absorption performances with the minimum reflection loss(RL)of-45.80 dB(at 1.7 mm thickness)and the maximum effective absorption bandwidth(E AB)of 3.6 GHz(at 1.5 mm thickness).After oxidation at 400-800℃,due to the coupling of conductivity loss and polarization loss,(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders can retain good EM wave absorption properties in a certain frequency range.In this paper,the effects of oxidation on EM wave absorption properties of high-entropy MAX phases were systematically investigated for the first time.This work manifests that high-entropy MAX phases are promising EM wave absorbing candidates and can maintain good EM wave absorption performances after oxidation.

Zr_(2)SeB and Hf_(2)SeB:Two new MAB phase compounds with the Cr_(2)AlC-type MAX phase(211 phase)crystal structures

The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases.However,MAB phases are generally referred to layered transition metal borides,while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics.In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides,two new MAB phase compounds Zr_(2)SeB and Hf_(2)SeB with the Cr_(2)AlC-type MAX phase(211 phase)crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work.First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr_(2)SeB and Hf_(2)SeB.Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering(SPS)furnace.The crystal structures of Zr_(2)SeB and Hf_(2)SeB were determined by a combination of the X-ray diffraction(XRD),scanning electron microscopy(SEM),and high-resolution transmission electron microscopy(HRTEM).The lattice parameters of Zr_(2)SeB and Hf_(2)SeB are a=3.64398Å,c=12.63223Åand a=3.52280Å,c=12.47804Å,respectively.And the atomic positions are M at 4f(1/3,2/3,0.60288[Zr]or 0.59889[Hf]),Se at 2c(1/3,2/3,1/4),and B at 2a(0,0,0).And the atomic stacking sequences follow those of the Cr_(2)AlC-type MAX phases.This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.

A novel medium-entropy(TiVNb)_(2)AlC MAX phase:Fabrication,microstructure,and properties

Medium-or high-entropy materials have great potential for applications due to their diverse compo-sitions and unexpected physicochemical properties.Herein,a novel medium-entropy(TiVNb)_(2)AlC was synthesized via hot pressing at 1400℃from three individual M_(2)AlC(M=Ti,V,Nb)MAX phases.The microstructure of(TiVNb)_(2)AlC was characterized from the microscale to the atomic scale by scanning electron microscope microscopy(SEM),scanning transmission electron microscopy(STEM),and energy dispersive spectroscopy(EDS).The results showed that Ti,V,and Nb atoms were fully solid-soluble in the M-sites of the M_(2)AlC MAX phase.Compared with three individual MAX phases,the thermal conduc-tivity of(TiVNb)_(2)AlC was reduced greatly in the temperature range of 293-1473 K,and its mechanical properties(including Young’s modulus,Vickers hardness,and bending strength)were all increased due to the solid solution strengthening and electronic mechanism.

MAX phase forming mechanism of M-Al-C(M=Ti,V,Cr)coatings:In-situ X-ray diffraction and first-principle calculations

The interesting hybrid properties of ceramics and metals induced by unique nano-laminated structures make the M_(n+1)AX n(MAX)phase attractive as a potential protective coating for vital structural compo-nents in harsh systems.However,an extremely narrow phase-forming region makes it difficult to prepare MAX phase coatings with high purity,which is required to obtain coatings with high-temperature anti-oxidation capabilities.This work describes the dependence of the phase evolution in deposited M-Al-C(M=Ti,V,Cr)coatings as a function on temperature using in-situ X-ray diffraction analysis.Compared to V_(2)AlC and Cr_(2)AlC MAX phase coatings,the Ti_(2)AlC coating displayed a higher phase-forming tempera-ture accompanied by a lack of any intermediate phases before the appearance of the Ti_(2)AlC MAX phase.The results of the first-principle calculations correlated with the experience in which Ti_(2)AlC exhibited the largest formation energy and density of states.The effect of the phase compositions of these three MAX phase coatings on mechanical properties were also investigated using ex-situ Vickers and nano-indenter tests,demonstrating the improved mechanical properties with good stability at high temperatures.These findings provide a deeper understanding of the phase-forming mechanism of MAX phase coatings to guide the preparation of high-purity MAX phase coatings and the optimization of MAX phase coatings with expected intermediate phases such as Cr_(2)C,V_(2)C etc.,as well as their application as protective coat-ings in temperature-related harsh environments.

Effect of Sn doping concentration on the oxidation of Al-containing MAX phase(Ti_(3)AlC_(2))combining simulation with experiment

Sn doping is usually adopted to prepare Ti_(3)AlC_(2)in mass production because it can reduce the synthesis temperature while increasing the phase purity.However,excessive Sn doping usually deteriorates the oxidation resistance of Ti_(3)AlC_(2).Therefore,an appropriate Sn doping concentration is a vital issue.In this work,the effect of Sn doping concentration on the oxidation behavior of Ti_(3)AlC_(2)was systematically investigated by combining theoretical calculations and experimental methods.Density function theory calculations suggest that the oxygen adsorption mechanisms for the(001)surface of Ti_(3)AlC_(2)with and without Sn doping are similar,and Ti-O bonds are always preferentially formed.The molecular dynamics simulation further indicates that Al atoms have a faster diffusion rate during the oxidation process.Therefore,a continuous Al_(2)O_(3)layer can form rapidly at high temperature.Nevertheless,when the Sn doping concentration exceeds 10 mol%,the continuity of the Al_(2)O_(3)layer is destroyed,thereby impairing the oxidation resistance of Ti_(3)AlC_(2).Furthermore,oxidation experiments verify the above results.The oxidation mechanisms of Ti3AlC2 with different Sn doping concentrations are also proposed.

Recent progress in additive manufacturing of bulk MAX phase components:A review

The MAX phases are a group of layered ternary,quaternary,or quinary compounds with characteristics of both metals and ceramics.Over recent decades,the synthesis of bulk MAX phase parts for wider engineering applications has gained increasing attention in aerospace,nuclear,and defence industries.The recent adoption of additive manufacturing(AM)technologies in MAX phase fabrication is a step forward in this field.This work overviews the recent progress in additive manufacturing(AM)of bulk MAX phases along with the achieved geometric features,microstructures,and properties after briefing the conventional powder sintering methods of fabricating MAX phase components.Critical challenges associated with these innovative AM-based methods,including,poor AM processability,low MAX phase purity,and insufficient geometric accuracy of the final parts,are also discussed.Accordingly,outlooks for the immediate future in this area are discussed based on the optimization of present fabrication routes and the potential of other AM technologies.

Topotactic transition of Ti_(4)AlN_(3) MAX phase in Lewis acid molten salt

MAX phases and its derived two-dimensional MXenes have attracted considerable interest because of their rich structural chemistry and multifunctional applications.Lewis acid molten salt route provides an opportunity for structure design and performance manipulation of new MAX phases and MXenes,Although a series of new MAX phases and MXenes were successfully prepared via Lewis acid melt route in recent years,few work is explored on nitride MAX phases and MXenes.Herein,a new copper-based 413-type Ti_(4)CuN_(3)MAX phase was synthesized through isomorphous replacement reaction using Ti_(4)CuN_(3)MAX phase precursor in molten CuCl2.In addition,it was found that at high temperature Ti4N3Clx MXene will transform into two-dimensional cubic TiNa nanosheets with improved structural stability.